With the ever-increasing public appetite for faster and more reliable connectivity, different systems and modulation schemes have been developed. For example, multimedia communications, such as video transmission, are becoming more and more popular and require high user data rate for good quality of viewing. Presently, Wireless Local Area Networks (WLANs) such as those using IEEE 802.11 standards are capable of providing data rates up to 54 Mbps and higher data rates are expected to be used in the future.
Multi-carrier systems such as those based on filtered multi-tone (FMT) and orthogonal frequency division multiplexing (OFDM) are popular technologies due to their ability to support high data rates and their robustness to multi-path interference. FMT techniques, for example, are able to activate and deactivate individual sub-carriers and the FMT spectrum is highly contained within each sub-band for each slot. This permits FMT techniques to be natural candidates for secondary systems, such as spectrum-sensing cognitive radio systems, in which adjacent channels may be occupied suddenly with relatively strong signals.
It is important to contain the radio frequency (RF) energy of signals both temporally and in the frequency domain. Some multi-carrier systems, such as OFDM systems, contain the RF energy within the communication band and avoid causing harmful interference to adjacent users in the RF band by applying frequency-domain windowing to the signal to limit adjacent channel interference. Other multi-carrier systems, such as scalable advanced modulation (SAM) systems, apply time-domain windowing to each slot to contain the RF energy within the current slot and out of the next slot.
In the frequency domain, other users in the adjacent RF bands will generate inter-carrier-interference (ICI) into both edges of the system's band. Frequency-domain windowing also generates ICI at both edges of a system's RF band. In the time domain, adjacent time slots generate some inter-symbol interference (ISI) to both ends of the current slot. In ISI, successive symbols blur together so that each symbol interferes with a subsequent symbol. ISI is usually caused by multipath propagation. Time-domain slot windowing can also cause ISI of symbols of both ends of a time slot.
All of these effects result in the downgrade of system reliability. It would thus be desirable to provide a more reliable technique for transmission of the data between a communication device and base station or between individual communication devices.